Hadoop is Big Data platform which stores and processes the data over distributed clusters.
In Hadoop we have NameNode and Data Nodes. Name Nodes is responsible for keeping track of all the meta-data about the data and stored on the data nodes in the cluster.
In the system there is one NameNode + one secondary node and multiple data nodes.
NameNodes provides the details of the data stored in data nodes to the client and then through this information data is accessed.
In the Hadoop HDFS files are stored in hierarchical system and meta data is manged at NameNode.
Check more at Big Data technology tutorials.